In the radio transmission art, it is well known that the effective length of the antenna must be "matched" to the transmission wave length. This is understood to mean that for a given wave length of radio transmission, the antenna must have an optimum length. Thus, in some transmission systems, it is the practice to vary the length of the antenna in order to match it to the transmission.
However, in the great majority of radio transmission systems, this is not practical. What is in effect done is that an antenna of a predetermined average length is provided, and an electronic matching circuit is provided between the transmitter and the antenna, so that, the apparent length of the antenna can be matched to the wave length actually being transmitted. Again, this is relatively well known.
For the purposes of this explanation the term "radio transmission system" is deemed to include either the radio transmitter, or receiver, or both. Matched antennas are often required for both for transmitting and receiving.
In the marine radio transmission art, one of the complicating factors is that it is highly desirable to provide what are known as "whip" antennas. A whip antenna is defined as an antenna which is fastened only at its base, and is essentially free-standing, without the intervention of any supports such as guy wires or the like.
In order to withstand the motion of a marine vessel at sea, particularly during high winds and rough weather, such whip antennas must have a certain degree of flexibility, so as to permit them to withstand the extreme motion of the vessel itself.
Some particularly effective forms of whip antenna design are described in U.S. Pat. Nos. 3,725,944, 4,300,140, and 4,500,888, all of which have been assigned to Valcom Ltd.
These whip antennas are constructed of fibreglass, and the longer versions of such antennas are constructed in sections which can be put together, somewhat in the manner of a fishing rod. Conductors are located within the body of the antenna carrying the radio signal up or down the length of the antenna.
Such whip antennas, made in accordance with these patents and have proved to be highly effective in use, and are widely used for marine purposes, particularly by military and naval vessels in various countries of the world.
However, due to the increasing crowding of the air waves by an increasing volume of radio signals, and also to a certain extent due to the requirements of security, it is desirable to provide for a radio transmission system on a marine vessel which will operate over a relatively wide band width. Desirably, such a radio transmission system may be capable of operating over band widths of from two megahertz to thirty megahertz. At present, it is not possible in this wide frequency range to provide a single antenna which can be matched to all of the transmissions over this wide band width. In the past, when somewhat narrower band widths may have been satisfactory, it was the practice to provide two pairs of whip antennas on a vessel. One pair would be mounted towards the bow, and the other pair of antennas mounted towards the stern of the vessel, in a typical case. By a suitable selection of the lengths of the antennas in the two pairs, it was possible to provide a reasonable degree of matching over the then acceptable band width for transmission and/or reception.
Alternatively one pair might be used for transmitting, and the other pair for receiving.
However, increasingly greater band widths are now required for transmissions, and greater and greater demands are made on the transmission systems, both for greater range and also for clarity of transmission, and for security.
It is also desirable to be able to switch bands relatively quickly, and this band switching may take place even within the space of a single message transmission.
When two pairs of antennas were regarded as satisfactory, it was the practice to provide one pair of shorter antennas and one pair of longer antennas. The antennas in their respective pairs acted as dipoles, so that they could be matched to handle a reasonable range of band widths of transmission or reception, within the limitations of their own lengths.
However, as mentioned above, due to the increasingly strict demands being made upon such radio transmission systems, it is no longer adequate to provide simply two pairs of whip antennas for any one radio system.
Requirements now call for the provision of three separate pairs of whip antennas. Each pair of antennas must be securely and substantially permanently attached at its location, i.e. to the vessel, and, bearing in mind the complexity and amount of other equipment which is also carried on such vessels, it has become increasingly difficult to find suitable locations for positioning the three pairs of antennas, particularly on smaller vessels. While there is relatively little problem in locating one pair of antennas near the bow, and another pair towards the stern, it is generally necessary to mount the third pair of antennas more or less amidships to the vessel.
The antennas must be mounted so that the two antennas in any one pair are no more than about ten feet apart from one another, in order to achieve the desired dipole effect.
While this does not present any serious problems, with the antennas mounted towards the bow and towards the stern, it does present problems with regard to the pair of antennas which are now required to be mounted amidships. Usually, the vessel will have one or more smoke stacks, emitting fumes or exhaust from the engines, and usually the smoke stack will also be located amidships. The fumes or exhaust as they leave the smoke stack may well be at a temperature in the region of six hundred or more degrees Fahrenheit and contain harmful chemicals. If antennas made of glass fibre reinforced resin materials are placed in close juxtaposition to the smoke stack, they will be unable to withstand these high temperatures and chemical emissions and the resin material will soon be degraded.
An additional, although not so serious a problem, was presented by the degree of flexibility incorporated in antennas made of glass fibre reinforced resins. Antennas made of such material were capable of relatively extreme degrees of deflection in high winds or during violent motion of the vessel in rough water. Depending upon where the antennas were placed, and how close to the smoke stack the antennas were located, it was conceivable that during extreme weather the antennas might contact the smoke stack and be damaged.
Accordingly, it is desirable to provide antennas which are both resistant to chemical emissions and to much higher temperature gases than are antennas made of glass fibre reinforced material, and which are also possessed of a higher degree of stiffness, making them more resistant to bending during extreme weather conditions. Antennas having such improved physical properties can then be mounted immediately alongside the smoke stack of a vessel, without fear of damage due to chemicals or to high temperature gases, and without fear that they will interfere with the smoke stack during extreme weather.